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Warren Blier

Abstract

The primary objectives of this study were 1) to test the ability of a high-resolution (40 km) limited-area model to successfully simulate a meso-α-scale case of polar airstream cyclogenesis, 2) to examine the effects of various physical conditions and dynamical processes on the storm development, and 3) to examine the evolving structure of the system. Principal findings were as follows.

  1. The control experiment, which utilized a 40-km horizontal grid increment, a Kuo-type cumulus parameterization, and surface sensible and latent heat fluxes, produced a small-scale cyclone with a central pressure of 996 mb at hour 24 (as compared with a subjectively analyzed pressure of 994 mb) with the position of the low center within 100 km of the analyzed location.
  2. Cyclogenesis, albeit significantly weaker, did occur in an adiabatic simulation. This appeared to result from the adiabatic forcing associated with a migrating upper-level short-wave trough.
  3. Strong ascent in the control simulation occurred near the surface low center and in a narrow plume on the warm-air side of a baroclinic zone that developed over the Bering Sea downwind of the polar ice sheet. This zone appeared structurally similar to that of the intense warm-frontal zone seen in explosive cyclones (near the low center). Removal of the effects of latent heat release significantly diminished the intensity of this rising motion and resulted in an expansion of its horizontal scale. A simulated increase in the sea surface temperature resulted in stronger rising motion and in a smaller and more intense cyclone.
  4. Surface fluxes and the attendant latent heat release in the plume of rising motion played a significant role in the intensification of the cyclone.
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Warren Blier

Abstract

Significant downslope wind and warming events periodically occur along a short segment of the southern California coast in the vicinity of Santa Barbara. This region is characterized by a unique mesoscale topography:over a length of about 100 km the coastline is oriented approximately west–east, with the adjoining narrow coastal plain bounded by a steeply rising (to elevations greater than 1200 m) and coast-parallel mountain range.

Called Sundowner winds because they often begin in the late afternoon or early evening, their onset is typically associated with a rapid rise in temperature and decrease in relative humidity. In the most extreme Sundowner wind events, wind speeds can be of gale force or higher, and temperatures over the coastal plain, and even at the coast itself, can rise significantly above 37.8°C (100°F). In addition to causing a dramatic change from the more typical marine-influenced local weather conditions, Sundowner wind episodes have resulted in significant property and agricultural damage, as well as extreme fire danger. They have, in fact, been associated with many of the most destructive conflagrations that have occurred in the Santa Barbara region.

In the present study, three different Sundowner wind episodes are examined. These include midsummer and midautumn events primarily manifested by extremely warm temperatures, and a winter season event notable for its damaging winds. The associated meteorological conditions are examined, and possible physical mechanisms responsible for these episodes are discussed. In at least two of the three cases considered here, mountain wave development appears to have played a significant role.

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Todd P. Mitchell and Warren Blier

Abstract

Rotated principal component (RPC) analysis, subject to the varimax criterion and including area weighting, is applied to a 58-yr record (1931–88) of monthly- and seasonal-mean Climatic Division precipitation anomalies for the contiguous United States to document wintertime precipitation variability in the region of California. Rotated principal components (time series) derived from this analysis are related to anomalies of seasonal-mean global sea surface temperature, and monthly mean Northern Hemisphere 500-hPa geopotential height and sea level pressure (SLP).

Wintertime seasonal-mean precipitation in California is captured by two RPCs. The first RPC documents coherent precipitation anomalies centered in northern California, Oregon, southern Idaho, and eastern Washington, and explains the largest portion of area-averaged variance of any of the patterns in the decomposition. A second RPC captures coherent precipitation variability in the south coast and southeast desert regions of California, southern Nevada, southern Utah, and northern Arizona. Fluctuations in the first RPC correlate poorly with Pacific Ocean SST anomalies. However, wet winters in the region of the second RPC correlate modestly with simultaneous cool western subtropical Pacific Ocean SST anomalies and weakly with warm SST anomalies over a broad region of the central and eastern tropical Pacific. The spatial scale of the tropical SST correlations and the prominent multidecadal timescale signal of the RPC are consistent with ENSO fluctuations on this timescale influencing southern California precipitation.

Consistent with the results of earlier studies, significant correlations are found between California wintertime monthly mean precipitation variability and regional 500-hPa geopotential height and SLP anomalies. Linear regression analysis is used to construct estimates of the total 500-hPa geopotential height and SLP fields (climatology plus anomaly) that are representative of the extreme wet and dry California winter months; these are then compared with the observed conditions in the individual extreme months. Several different flow patterns appear capable of producing anomalously large monthly precipitation totals in California.

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Warren Blier and Roger M. Wakimoto

Abstract

The structural evolution of the extratropical cyclone that occurred during intensive observation period 5 (IOP 5) of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) is presented. In the present paper, the focus will be on the synoptic-scale development of the cyclone and the mesoscale frontal structure in the vicinity of the surface low center at a time early in its period of rapid deepening. Within this context the much higher resolution analyses of the three-dimensional wind field of the circulation center are discussed in the companion paper.

The surface low center appeared initially as one of at least two mesoscale circulation centers along a preexisting low-level frontal zone approximately aligned with the Gulf Stream. Development occurred in association with the approach of a synoptic-scale upper-level trough from the west. As in the ERICA IOP 4 cyclone, a bent-back warm front-representing the extension of the warm front to the west of its point of intersection with the cold front-developed early in the period of rapid deepening. The surface low center was found to lie along this front and was associated with a significant meso-β-scale circulation. A secondary cold front developed out of the bent-back front to the south of the low center, while the antecedent primary cold front showed evidence of fracture near its juncture with the warm front. This frontal weakening appeared to be associated with a diffluent low-level wind field and with the intrusion of a swath of warm and dry air.

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Richard J. Reed and Warren Blier

Abstract

A case study is presented of the development within a polar air stream of a comma-shaped cloud pattern (comma cloud) and associated small surface cyclone. The disturbance is traced from the time of its development over the eastern Pacific Ocean until it moves inland over California as a mature system. The first sign of the development was the appearance of a region of enhanced convection in the northwesterly flow behind an amplifying large-scale trough and ahead of an embedded short-wave trough. As the development progressed, the cloud field expanded and assumed the comma shape. The head, located on the cold, cyclonic-shear side of the jet stream, was composed of large, deep convective elements that merged sequentially. The tail, located on the warm, anticyclonic-shear side, was composed of shallower, more stratiform clouds. The surface low center formed within the comma head during the phase of rapid organization and strengthening of the convection.

Detailed surface and upper air analyses of the system over California during the mature stage revealed that the disturbance was at this stage associated with a weakly baroclinic region in the lower and middle troposphere and was capped by a strong upper-tropospheric frontal zone. Although no change of air mass accompanied the passage of the comma cloud, the disturbance did exhibit frontal characteristics at the surface as it passed stations near the southern California Coast. Precipitation within the mature cloud band tended to be stratiform, with some rather large precipitation totals reported.

The quasi-geostrophic omega equation is employed to elucidate the processes involved in the development of the comma cloud. A qualitative analysis indicates the likely importance of small static stabilities in enhancing the effect of the relatively modest positive vorticity advection. The possible importance of latent heat release on the development of the system is also discussed. Finally, the main results of this paper and a companion paper are summarized in the form of a schematic diagram.

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Richard J. Reed and Warren Blier

Abstract

In a companion paper the authors presented a case study of the development of a comma-shaped cloud pattern and associated small cyclone that formed in a cold air mass over the eastern Pacific. This paper confirms the reproducibility of the previous analysis by documenting a second, similar case. A schematic model of comma cloud development appearing in the companion paper is based in part on this second example.

An added feature of the present paper is a detailed examination of the physical processes responsible for the enhanced convective activity that occurred during the intensifying stage of the disturbance. It is found that the lifted index decreased markedly in the vicinity of the developing comma and that the decrease was associated with warming and moistening of the surface air and cooling of the air at 500 mb. Sensible and latent heat fluxes from the surface of approximately 100 and 300 W m−2, respectively were essential to the low-level warming and moistening. Amplification of an upper-level long-wave trough contributed to the cooling aloft.

As the system came ashore in southern California, convective activity was much more severe in this case than in the previous one. A likely important factor in this difference was the greater warmth of the coastal waters in this (November) case than in the earlier (March) case.

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Warren Blier and Karen A. Batten

Abstract

Climatological analyses of tornado occurrence in the state of California for the period 1950–1992 are presented. In constructing these analyses, the official historical record of California tornadoes was supplemented and corrected with tornado reports from other sources. In corroboration of the results of the few previous studies of California tornadoes, the distribution of tornadic events across the state is found to be very uneven; in particular, a relatively small area of south-coastal California has an incidence of tornadoes (per unit area per unit time) comparable to regions within the midwestern United States. Other subregions of the state with an enhanced incidence of tornadoes are also identified; these include a large portion of the Central Valley (which comprises the Sacramento and San Joaquin Valleys), the north-central coastal region (including the San Francisco and Monterey Bay areas), and a part of the vast southeast desert region. Annual and diurnal distributions of tornadoes in each of these areas are examined. Tornadoes in the southeast desert region are found to occur primarily during the warm season, while those in the other three identified subregions occur primarily during the cool season. Peak incidence generally occurs during the afternoon, though the diurnal distribution is complex in the two coastal regions. The average tornado in California is weaker and has a shorter path width and pathlength than the average tornado in the contiguous United States; however, the preferential occurrence of tornadoes in areas of California that are moderately-to-densely populated makes them a source of significant concern.

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Roger M. Wakimoto, Warren Blier, and Chinghwang Liu

Abstract

Observation taken during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) have permitted analyses of explosive oceanic cyclogenesis of unprecedented detail. The most intense of the cyclones that occurred during this experiment was that of intensive observing period 4 (IOP 4). This storm not only contained the lowest sea level pressure ever observed south of 40°N in an extratropical cyclone over the Atlantic but was well-sampled by the specially deployed observing systems (aircraft, airbone Doppler radar, dropwindsondes, and buoys. This paper presents detailed analysis of this case. The primary issues addressed here are 1) the finescale structure of the fronts, and 2) the structure and organization of the associated precipitating features.

Both of these issues have previously been investigated primarily through numerical simulation of various cases of intense cyclogenesis. Analysis of the resulting model output has indicated a structural evolution of such cyclones that departs significantly from that described by the Norwegian cyclone model. Diagnosis of the output has indicated that latent heat release plays a significant role, both in the cyclone intensification and in the evolution of the associated fronts. The detailed in situ observations in the present case allow for observational evaluation of the attendant conclusions of these prior modeling studies.

Principal findings include:

1) Confirmation of the existence of a “bent-back%rdquo; warm front wrapping to the west around the cyclone center; the frontal structure is very different from that of the occluded front that would here by analyzed according to the Norwegian model.

2) The presence of an extremely sharp warm front, with Kelvin-Helmholtz waves and an intense line of convection found along the front.

3) A continuous extension of convective activity along the cold front to the point of intersection with the warm front, with no evident "fracture” zone.

4) The presence of only scattered convective cells along and to the north of the bent-back warm front.

5) A significant displacement between the cold front and the main cloud band. The cold front lay along a narrow line of intense convection well to the rear of the main comma-shaped cloud mass evident in the satellite imagery.

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Warren Blier, Stanley Keefe, Wilson A. Shaffer, and Sung C. Kim

Abstract

Within the period of the historical record there have been several occurrences of extensive damage from storm-surge-related coastal flooding in the region of Nome, Alaska. The most recent of these events, although by no means the most destructive, occurred in association with the storm of 5–6 October 1992. Despite the small population of Nome (approximately 4000 people), total damage costs exceeded $6 million.

The research into the nature and causes of such flooding events has focused on this October 1992 case. The authors have, however, also examined a weaker, shorter-duration event that occurred on 20 August 1993 and, for contrast, a case in September 1993 where a sustained offshore wind transported water out of Norton Sound. Tide gauge data from Nome were used to quantitatively assess the associated changes in water level, and meteorological analyses were utilized to examine the associated synoptic-scale circulations and their evolution.

In addition, numerical modeling experiments were conducted using an extratropical storm surge model. (A version of this model is operational for the east coast of the United States.) Hindcasts of phase and amplitude for the October 1992 and September 1993 events agreed well with observations. Simulations of the shorter-duration August 1993 event were in poorer agreement with observations and indicate several possibilities for future improvement of the performance of the surge model: enhancement of the horizontal and temporal resolution of the model domain; more accurate input sea level pressure and wind data; and improvements to the surge model itself (e.g., inclusion of sea ice). Overall, however, results indicate that recent operational implementation of the model should be of significant benefit to coastal forecasters.

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John P. Monteverdi, Warren Blier, Greg Stumpf, Wilfred Pi, and Karl Anderson

Abstract

On 4 May 1998, a pair of tornadoes occurred in the San Francisco Bay Area in the cities of Sunnyvale (F2 on the Fujita scale) and Los Altos (F1). The parent thunderstorm was anticyclonically rotating and produced tornadoes that were documented photographically to be anticyclonic as well, making for an extremely rare event. The tornadic thunderstorm was one of several “pulse type” thunderstorms that developed on outflow boundaries on the left flank of an earlier-occurring thunderstorm east of San Jose. Satellite imagery showed that the tomadic storm moved northwestward along a sea-breeze boundary and ahead of the outflow boundary associated with the prior thunderstorms. The shear environment into which the storm propagated was characterized by a straight hodograph with some cyclonic curvature, and by shear and buoyancy profiles that were favorable for anticyclonically rotating updrafts. Mesoanticyclones were detected in the Monterey (KMUX) radar data in association with each tornado by the National Severe Storm Laboratory's (NSSL) new Mesocyclone Detection Algorithm (MDA) making this the only documented case of a tornadic mesoanticyclone in the United States that has been captured with WSR-88D level-II data. Analysis of the radar data indicates that the initial (Sunnyvale) tornado was not associated with a mesoanticyclone. The satellite evidence suggests that this tornado may have occurred as the storm ingested, tilted, and stretched solenoidally induced vorticity associated with a sea-breeze boundary, giving the initial tornado nonsupercellular characteristics, even though the parent thunderstorm itself was an anticyclonic supercell. The radar-depicted evolution of the second (Los Altos) tornado suggests that it was associated with a mesoanticyclone, although the role of the sea-breeze boundary in the tornadogenesis cannot be discounted.

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